C.I. Direct Blue 6, a commercially available textile dye, has been shown to produce hepatocellular carcinoma in rats fed diets containing from 1,500 to 3,000 ppm of test chemical. The dye is derived from benzidine, a known hepatocarcinogen in rats, and when administered orally to rats the dye is metabolized to the carcinogen. Hepatocellular carcinomas induced by the dye appeared after only five weeks of treatment, an onset time which is much shorter than has ever been reported for benzidine-induced carcinomas in rats. This observation raises the question of whether the dye-induced carcinomas were mediated through the dye per se, benzidine produced through the biotransformation of the dye, or some other metabolite. When considered on a molar basis, the benzidine equivalents of the dye were far in excess of any dosage levels of benzidine reported in the literature. The objective of this study is to compare the toxicities and carcinogenicities of benzidine and Direct Blue 6 (in molar equivalent doses with respect to benzidine).